The present invention relates in general to a phosphor screen, and in particular to a phosphor screen that corrects luminous non-uniformity in an image intensifier and to a method of making such a screen.
Image intensifiers are well known for their ability to enhance night-time vision. The image intensifier multiplies the amount of incident light received by it to produce a signal that is bright enough for presentation to the eyes of a viewer. For instance, the U.S. military uses image intensifiers during night-time operations for aiming at targets that otherwise would not be visible.
A prior art image intensifier 10 is shown in FIG. 1. An objective lens 12 focuses electromagnetic radiation from a distant object 14 onto a photocathode 16. The electromagnetic radiation can be in the visible spectrum or in the invisible spectrum. The photocathode 16 converts the electromagnetic radiation into an electron emission. A microchannel plate (MCP) 18 amplifies the electron emission by increasing the electron density of the electron emission, which increases brightness. A phosphor screen 20, deposited on the back of a fiber optics element 22, converts the amplified electron emission into visible light. The fiber optics element 22 transmits the light to an observer 24. The object is inverted when it reaches the photocathode 16, and therefore the image provided by the phosphor screen 20 is also inverted.
In certain image intensifiers, it is desirable to invert the inverted image before it is viewed. One method of inverting the image is to follow the fiber optics element 22 with a lens assembly. A far more advantageous method, however, is to twist the fiber optics element 22. The twisted fiber optics element 22, more commonly known as a "twister", is created by twisting a fiber optic bundle such that the input image, which falls on the input plane, is inverted by 180 degrees when the input image finally reaches the output plane 23.
The twister offers several advantages over the lens assembly. For instance, the twister consumes less space, because inversion can be accomplished over a shorter distance. The twister also offers a sharper image than the lens assembly. The twister is more convenient to use, since a fiber optics element 22 already forms a part of the image intensifier. And, unlike the lens assembly, the twister cannot be knocked out of alignment.
There is a disadvantage to twisting the fiber optics bundle, however. By twisting the bundle, a longer optical path is produced along the outer fibers than along the central fibers. The longer optical path in the longer fibers attenuates light more than the shorter optical path in the shorter fibers, thereby causing a gradient in luminosity between an edge of the twister and the center of the twister. For example, the transmission efficiency of an optical signal at the center of the twister may range between 75 and 80 percent, whereas the transmission efficiency of an optical signal at the edges may range between 55 and 60 percent. Therefore, the fiber optics transmits light having a gradient of luminosity.
The prior art is aware of apparatus methods and for correcting luminous non-uniformity caused by a twister. For instance, in U.S. Pat. No. 4,932,747, entitled "FIBER BUNDLE HOMOGENIZER AND METHOD UTILIZING SAME" and issued to Stephen Russell and George Imthurn on 12 Jun. 1990, a fiber optics element is employed to homogenize the non-uniform intensity profile of an excimer laser. An anti-reflective coating is deposited at the input faces of the input ends of the filaments, which form the fiber optics element. The input ends receive a beam from the excimer laser, and a uniform intensity profile is produced is provided at the output ends.
In U.S. Pat. No. 3,977,855, entitled "METHOD OF MAKING FIBER OPTIC DEVICE" and issued to Henry Cole on Aug. 31, 1976 and assigned to American Optical Corporation, fiber stretching in the twister is minimized. A fiber optics bundle, having its image-receiving and emitting faces arranged in identical geometrical patterns, is formed by heat softening its intermediate section, twisting the bundle by 180.degree. and then compressing the bundle in the axial direction.
In U.S. Pat. No. 4,656,562, entitled "OPTICAL INTEGRATOR MEANS FOR INTENSITY MODIFICATION OF GAUSSIAN BEAM" and issued to Paul Sugino on 7 Apr. 1987, and assigned to Santa Barbara Research Center, discloses an integrator rod that effectively inverts the cross-sectional intensity of a beam having a generally Gaussian intensity distribution. The integrator rod is made of an optically transmitting material and has a faceted entrance face and an exit face having a different geometry. The light beam has a generally uniform intensity distribution as it leaves the exit face of the rod.
In U.S. Pat. No. 4,360,372 entitled "FIBER OPTIC ELEMENT FOR REDUCING SPECKLED NOISE", issued to Roman Maciejko on 23 Nov. 1982 and assigned to Northern Telecom Limited, a fiber optics element has a bundle of filaments with different refractive indices. The refractive indices vary the lengths of the optical paths of the respective filaments.